Low-boiling hydrocarbons, such as methane, ethane, propane, butane, and iso-butane, are normally present in conduits that are used for the transport and processing of natural gas and crude oil. When varying amounts of water are also present in such conduits the water/hydrocarbon mixture is, under conditions of low temperature and elevated pressure, capable to form gas hydrate crystals. Gas hydrates are clathrates (inclusion compounds) in which small hydrocarbon molecules are trapped in a lattice consisting of water molecules. As the maximum temperature at which gas hydrates can be formed strongly depends on the pressure of the system, hydrates are markedly different from ice.
The structure of the gas hydrates depends on the type of the gas forming the structure: methane and ethane form cubic lattices having a lattice constant of 1.2 nm (normally referred to as structure I) whereas propane and butane from cubic lattices having a lattice constant of 1.73 nm (normally referred to as structure II). It is known that even the presence of a small amount of propane in a mixture of low-boiling hydrocarbons will result in the formation of type II gas hydrates which type is therefore normally encountered during the production of oil and gas. It is also known that compounds like methyl cyclopentane, benzene and toluene are susceptible of forming hydrate crystals under appropriate conditions, for example in the presence of methane. Such hydrates are referred to as having structure H.
Gas hydrate crystals, which grow inside a conduit, such as a pipeline, are known to be able to block or even damage the conduit. In order to cope with this undesired phenomenon, a number of remedies have been proposed in the past such as removal of free water, maintaining elevated temperatures and/or reduced pressures or the addition of chemicals such as melting point depressants (antifreezes). Melting point depressants, typical examples of which are methanol and various glycols, often have to be added in substantial amounts, typically in the order of several tens of percent by weight of the water present, in order to be effective. This is disadvantageous with respect to costs of the materials, their storage facilities and their recovery, which is rather expensive.
Another approach to keep the fluids in the conduits flowing is taken by adding crystal growth inhibitors and/or compounds, which are in principle capable of preventing agglomeration of hydrate crystals. Compared to the amounts of antifreeze required, already small amounts of such compounds are normally effective in preventing the blockage of a conduit by hydrates. The principles of interfering with crystal growth and/or agglomeration are known.
U.S. Pat. No. 6,905,605 describes a method for inhibiting the plugging of a conduit containing a flowable mixture comprising at least an amount of hydrocarbons capable of forming hydrates in the presence of water and an amount of water, which method comprises adding to the mixture an amount of a dendrimeric compound effective to inhibit formation and/or accumulation of hydrates in the mixture at conduit temperatures and pressures; and flowing the mixture containing the dendrimeric compound and any hydrates through the conduit.
Some of the hydrate inhibitors described above have properties that are undesirable under certain circumstances. For example, some of the hydrate inhibitors have a low cloud point temperature. Above the cloud point temperature the solubility of these polymeric inhibitors in water decreases drastically which can result in the precipitation of sticky polymer masses.
It would be advantageous to develop hydrate inhibitors that have a high enough cloud point so that the inhibitor does not become cloudy (begin to precipitate solids) under conditions where the hydrate inhibitors are used.